As technologies further advance, a variety of portable devices, such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like are powered by a power source having a USB port. The portable devices receive power from the power source through a USB cable inserted into the USB port.
The power source may be a wall adapter having the USB port. The wall adapter converts power from an alternating current (hereinafter “ac”) power supply to a regulated direct current (hereinafter “dc”) voltage. The wall adapter may include an electromagnetic interference filter (EMI) filter, a diode rectifier, an isolated dc/dc converter and the USB port providing an interface between the wall adapter and the USB cable.
The EMI filter of the wall adapter is used to suppress conducted EMI noise from the ac power supply. Two input terminals of the diode rectifier are connected directly to the two output terminals of the EMI filter. The diode rectifier is able to convert the input ac waveform to a pulsating dc waveform at the output terminals of the diode rectifier. A plurality of capacitors may be coupled between the two output terminals of the diode rectifier. The capacitors are used to attenuate the ripple of the pulsating dc waveform.
The isolated dc/dc converter may be implemented as a flyback converter. The flyback converter is derived from buck-boost topology. The flyback converter may be formed by a transformer, a switch, a diode and a plurality of output capacitors. The primary side of the transformer is connected in series with the switch. The secondary side of the transformer is connected in series with the diode. The transformer provides a voltage ratio from input to output. In addition, the transformer provides isolation. Furthermore, the flyback converter's transformer functions as a pair of coupled inductors for storing energy. When the switch is turned on, energy is stored in the winding of the transformer. The polarity dots of the transformer and the diode are arranged such that there is no energy transferred to the secondary side when the switch is on. The load current is maintained by the plurality of output capacitors. On the other hand, when the switch is turned off, the polarity of the transformer reverses. As a result, the output rectifier conducts and the energy stored in the transformer is transferred to the load as well as the output capacitors.